March 12, 2009

Honey, I Unshrunk The Fish

As we’ve mentioned before, one of the most unfortunate (but also most scientifically interesting) consequences of overfishing is that it can cause fish to shrink. Smaller fish are better able to slip through the holes in fish nets and therefore survive to pass on their genes rather than ending up as fish sticks. As a result, heavily-harvested fish populations—especially in places that have minimum net mesh size requirements designed to let a certain fraction of fish escape—tend to evolve toward smaller average body sizes. This is bad for both fishermen and fish eaters, given that larger fish tend to have more usable meat and fetch better prices.

So that’s the bad news, which regular Vine readers have already heard. The good news is that David Conover, the Stony Brook University marine ecologist who did some of the original research on fisheries-induced evolution, has published a new paper suggesting that the process of fish shrinkage may be more reversible than scientists originally thought. Conover started with a population of fish in which he had simulated fisheries-induced evolution by removing the larger 90 percent of the fish from each generation. He then switched to removing 90 percent of each generation at random, eliminating the selective pressure toward smaller body sizes. After five generations, the fish had regained about 50 percent of their length. Given their rate of recovery, Conover projects that the fish will be back to their original size after approximately twelve generations.

The reason not to get overly excited is that most commercially harvested fish species have fairly long generation times—Conover notes in the paper that three to seven years are typical. So even after the selective pressure on a fishery is relaxed, recovery could take decades. And getting rid of fisheries-induced selective pressure towards smaller fish means doing things like establishing no-fishing marine reserves that act as gene banks. In many places, that could turn out to be a real political challenge.